The present invention relates to a method and apparatus for the detection and location of subterranean anomalies, and particularly, to such method and apparatus for detecting tunnels or other void spaces within the earth.
Geological surveying by electromagnetic means has been done for many years. The techniques that have been used involved the detection of changes in the earth's resistivity by measurements in mutual coupling between a transmitter antenna (i.e., a long wire or small horizontal loop) and a receive antenna located on or near the earth's surface. This technique works well for large anomalies (i.e. gravel pits, large oil or ore deposits, etc.) and small anomalies located near the earth's surface. The technique involves an averaging of the earth's resistivity and how much an anomaly will change this total average. Another technique involves generation of parallel magnetic fields in separate conductors by driving them with out of phase currents. This method is exemplified by U.S. Pat. No. 1,660,774 issued to M. Voss et al. Another method involves the drilling of two boreholes parallel to one another and doing transmission line experiments. This technique is useful but expensive to implement considering the cost of drilling.